In communication systems, one may need a symbol to contain a certain received energy, usually termed Es, in order to correctly decode the information carried by the symbol. However, in some situations, one may also need the transmitted power per Hertz (Hz) to be below some ceiling. This is often required to avoid interfering with other signals in the same frequency band. One way to do this may be to widen the spectrum of the transmitted signal, i.e., to transmit the same energy over a wider bandwidth, in order to be able to meet such a constraint. For example, if one would like to transmit 10 kbps and would need somewhere in the 13-20 kHz range to send it, one may need to spread that power across, e.g., 1 MHz of bandwidth.
One known way to do this is direct-sequence (DS) spreading. In a DS spread-spectrum (DSSS) system, each symbol to be transmitted may be multiplied by a spreading sequence, which may be, for example, but is not limited to, a pseudo-random sequence (to be referred to as a “PN sequence” herein). As a result, for a particular symbol, one may multiply by, for example, an 80-chip PN sequence, and the resulting increase in the number of transitions (which may be in phase, frequency or amplitude or some other characteristic) between those chips may serve to widen the spectrum.
An issue in using DSSS is that it may make acquisition more difficult, and the longer the PN sequence used, the more difficult acquisition becomes. As a result, long training sequences may be needed to provide synchronization/acquisition for DSSS systems using long PN sequences. Therefore, it may be desirable to use another technique to shorten the length of the PN sequence needed or instead of using DSSS.